This invention relates to a replaceable, high current, draw-out fuseholder, having a unitary, cycloaliphatic, epoxy-glass, filament wound, tubular insulating member with resin rich surfaces disposed between fuse contacts. These fuseholders are used in pad mounted and submersible distribution transformers.
Replaceable, under oil expulsion fuses are generally used in high voltage systems to protect electric devices from fault currents, and are disclosed in U.S. Pat. No. 4,320,375 (Lien). There, the fuseholder includes a glass wound tube, impregnated with epoxy resin, covering an inner pressure tube of a nontracking, nonconducting material, such as polytetrafluoroethylene (Teflon). Both tubes are shown having about the same thickness. Hoop strengths of about 141 kg./cm.sup.2 (2000 psi) are mentioned. This composite, tubular, insulating structure is disposed between and fitted flush with two electrically conductive contacts having similar lengths and configurations. A metallic fuse element which will melt at a particular load current or temperature, to interrupt the circuit, extends through the interior of the hollow tubular structure between the contacts. The fuseholder is shown mounted in an open housing which is totally immersed in insulating oil. This type of fuseholder has disadvantages of relatively low hoop strength, and an outer surface containing exposed glass filaments which may cause copper tracking from mating housing contacts, during insertion or removal of the fuseholder, limiting its replaceability.
Similar type expulsion fuses, having a glass epoxy-Teflon pressure tube between threadedly mounted metal contacts of similar length and configuration, each having a diameter substantially greater than the pressure tube, are disclosed in U.S. Pat. No. 4,625,196 (Muench et al.) In this patent, primarily directed to the fuse assembly, both metal contacts have a beveled inner chamfer so that the pressure tube substantially "blends" into the contacts. In a modification of this design, U.S. Pat. No. 4,628,292 (Muench et al.) discloses a single layer, glass epoxy pressure tube between threadably mounted metal contacts, each having a diameter substantially greater than the pressure tube. In this patent, also primarily directed to the fuse assembly, one metal contact has a beveled inner chamfer and the other metal contact, which appears elongated, has a sharp inner edge, and contains both an inner pressure chamber and vent holes through the contact surface to the pressure chamber.
Earlier art had disclosed the use of epoxy resin impregnated glass fibers as tubular structures for a variety of fuse types. Canadian Pat. No. 704,315 (Cannady et al.) discloses such tubes, with encircling band members of epoxy resin impregnated material, such as cotton, nylon or Dacron, that could be easily machined to provide end threads. U.S. Pat. No. 3,184,829 (Shobert) discloses a compressed boric acid inner tube and a ground smooth outer tube of resin impregnated glass fiber braiding.
U.S. Pat. No. 3,911,385 (Blewitt et al.) discloses an outdoor, weather resistant fuse, which uses a melamine resin impregnated glass fiber tube, coated on its outer surface with a cycloaliphatic epoxy resin. The epoxy resin contains a flexibilizing agent, such as a mixture of polyazelaic polyanhydride and hexahydrophthalic anhydride, filler such as aluminum trihydrate or naturally occurring magnesite, and asbestos thickener-thixotroping agent. The outer coating is from 0.007 cm. (0.003 inch) to 0.051 cm. (0.020 inch) thick. A silicone rubber sealant is used in the joint between the tubular member and end terminals of similar length and configuration. No separate inner tubular member or coating is used, so melamine resin and glass fibers are exposed to the contaminating products of fuse link melt down. Problems associated with this design are possible carbonization of the glass fibers in the tube interior, and bonding of the epoxy resin and melamine resin at the coating interface. U.S. Pat. No. 3,979,709 (Healey, Jr.) discloses a fuse having a central tube made of thermoset resin impregnated glass fiber mat, having non-uniformly oriented fibers disposed between inner and outer tubular members made of thermoset resin impregnated glass fiber fabric.
In another area, feed tubes, used in extra high voltage circuit breakers, operating in an environment of SF.sub.6 gas subject to arcing, have been made using resin rich, flexible, cycloaliphatic epoxy resin surfaces. U.S. Pat. Nos. 3,828,000 and 4,102,851 (both Luck et al.) disclose mixtures of cycloaliphatic epoxy resin containing polyazelaic polyanhydride or hexahydrophthalic anhydride as a flexibilizer and curing agent, aluminum trihydrate, and either aluminum oxide Al.sub.2 O.sub.3 or short-fiber asbestos as an essential thixotropic agent. This mixture is disclosed as being coated on mandrels, cured, covered with filament wound glass fibers coated with the same mixture, recoated on top also with the same mixture, and finally cured to provide a flexible insulating surface.
Most such previously described structures have not been found to provide superior hoop strength, to withstand the high pressure shock wave created by the vaporization of the fuse element, and at the same time eliminate any cracks between various component layers, to provide superior dielectric properties when exposed to hot oil conditions, and to eliminate copper tracking of the outer surface upon removal or insertion against mating contacts; so that the fuseholder is highly replaceable. It is the object of this invention to solve such problems.